This invention relates to an apparatus for controlling a drive system for an industrial truck according to claim 1.
Industrial trucks which are intended to be given a large radius of action or a high efficiency are powered by internal combustion engines. Diesel engines or spark ignition engines powered by fuel gas are mainly used for this purpose.
It is known to adapt the number of revolutions and the torque of the internal combustion engine to the requirements made to the driving wheels by means of a hydrostatic transmission. A hydrostatic transmission usually comprises a hydraulic pump the delivery volume of which is variable and a hydraulic engine. If a differential mechanism is to be realized two hydraulic engines are used which, in turn, are variable in their delivery volumes, if required. The hydraulic pump usually is structured so as to allow four-quadrant operation, which means that powering and braking it is possible in the forward and backward directions.
It is known from the company brochure xe2x80x9cPOCLAIN HYDRAULICS PHxe2x80x9d of February, 1998 to employ an electronic control which coordinates the number of revolutions of the internal combustion engine and the delivery volume set for the hydraulic pump with the position of the accelerator and the brake pedal. The delivery volume is varied by a servo-cylinder by means of a lever. The servo-cylinder is acted on by hydraulic oil via two electrically operated proportional valves from two sides. The supply unit of the varing mechanism comprises the feed pump and a pressure relief valve.
As a rule, monotonically increasing characteristics are preset which assign a number of revolutions of the internal combustion engine and a delivery volume and, hence, a volume flow to each accelerator position. For a flexible drivability, the target values are reached via ramps which limit the amounts of such variations. This also restricts the acceleration of the industrial truck to an admissible rate. Admissible rates are determined via the power potential of the internal combustion engine and the admissible pressure of the hydraulic system.
If vehicles are loaded the hydrostatic transmission might happen to require the internal combustion engine to deliver a torque that it is unable to deliver. To prevent the internal combustion engine from being xe2x80x9cstalledxe2x80x9d it has become known from the company brochure xe2x80x9cElektronisch geregelter Fahrantrieb fur Gabelstaplerxe2x80x9d by Peter Dschida of Brueninghaus Hydromatik GmbH to take appropriate countermeasures for an electronically controlled travelling mechanism for fork-lift trucks. It is detected here that the internal combustion engine is incapable of achieving the target number of revolutions. As a consequence, the delivery volume of the hydraulic pump is reduced, which relieves the internal combustion engine. A hydraulic pump having a constant delivery volume for the lifting and other functions of the industrial truck frequently is connected, in addition to a geared pump, to the shaft of the internal combustion engine. The electronic control presets a larger number of revolutions for the internal combustion engine if lifting speeds are larger. However, to prevent the vehicle from getting accelerated the delivery volume of the hydraulic pump of the travelling mechanism is reduced accordingly.
The electronic control which is known has the disadvantage that the number of revolutions of the internal combustion engine is dependent on the travelling speed. The measure which as is known makes it impossible for the internal combustion engine to get stalled is relatively sluggish and pressure-induced vibrations, a juddery motion of the vehicle, and a non-optimal utilization of the power potential of the internal combustion engine may readily occur. The inflexible delivery volume preset causes the vehicle to start up with a jerk and causes hydraulic vibrations if disturbing factors act on it.
It is the object of the invention to provide an apparatus for controlling a drive system for an industrial truck which leads to minimum fuel consumption, only applies admissible torques to the internal combustion engine, and improves its drivability.
The object is attained by the features of claim 1.
The inventive apparatus uses a cascaded control. The cascade comprises an overlying speed control circuit and an underlying pressure control circuit. The pressure controller compares a transmission pressure measured to a preset pressure (a desired-pressure value) with the pressure controller speedily varying the hydraulic pump delivery volume so as to cause the control error to disappear. The overlying speed controller compares the travelling speed desired to the one measured, calculating a desired motive force therefrom. The motive force which is desired is converted, in a way which yet is to be described, into a required pumping pressure which is imposed onto the pressure controller as a desired-pressure value.
A characteristic, which indicates an optimal-consumption speed for a given power, exists for each internal combustion engine. The driving power required may be calculated from the travelling speed and the motive force. This allows to preset a desired number of revolutions for an internal combustion engine from the characteristics stage in accordance with the characteristic chosen. This one will then be imposed onto a common speed controller for an internal combustion engine.
Since the number of revolutions of the internal combustion engine is chosen according to the power requirement an optimal-consumption operation may be ensured. Pressure can be limited to the values admitted by the components at any point of operation. This makes it possible to prevent the engine from getting stalled and to set the limitation to a maximum value for the transmission pressure. In addition, the inventive apparatus improves the drivability and levels out vibrations caused by pressure. The latter are avoided by dimensioning the pressure controller in a proper way.
If the internal combustion engine drives another hydraulic pump, e.g. for the lifting system, the number of revolutions of the internal combustion engine requires to be chosen higher to achieve a sufficient lifting speed. As a result, the number of revolutions ceases to be completely at the minimum of consumption. However, an increase in travelling speed does not take place because this does not influence the presetting effected for of the motive force and the hydraulic pressure. The pressure controller automatically reduces the delivery volume in order not to increase the pressure.
In the inventive apparatus, the engine""s characteristics are derived from the rpm-torque-efficiency characteristic diagram of the internal combustion engine. In an aspect of the invention, an rpm-torque-efficiency characteristic diagram for the pump is stored in the characteristics stage for being taken into account in determining the desired rpm value. Similarly, in another aspect of the invention, a travelling speed-torque-efficiency characteristic diagram for the hydraulic engine can be stored in the characteristics stage for being taken into account in determining the desired rpm value.
A sensor may be disposed on the wheel or the hydraulic engine to determine the number of revolutions for the inventive apparatus and the speed transducer may determine the speed from the number of revolutions and the radius of the wheel. Alternatively, an rpm sensor may measure the number of revolutions of the internal combustion engine and calculate the speed therefrom and from the delivery volume of the pump (which corresponds to the transmission gear ratio) while taking into account the radius of the wheel.
At least one pressure sensor may be provided, as a pressure transducer, at the outlet of the pump. In another aspect of the invention, the pump may have hydraulic variation inlets which are connected to the pressurized side of an auxiliary pressure source via a proportional valve each. The control input of the proportional valves is acted on by the desired value of the desired-value transducer.
The characteristic which is employed may be different depending on the operating state of the inventive apparatus or internal combustion engine. Therefore, an aspect of the invention provides that the characteristics stage should have several sub-stages storing a characteristic each and a switch-over device is provided to change over to a sub-stage desired. The switch-over device may have a manually operable switch or may be actuated in dependence on the position of the accelerator or its speed of actuation.
In an aspect of the invention, the number of revolutions of the internal combustion engine, the volume displaced by the hydraulic pump, and the number of revolutions of the driven wheel may be resorted to for a determination of the actual-pressure value in the hydrostatic transmission. Alternatively, the actual-pressure value may be determined from the torque of the internal combustion engine and the volume displaced by the hydraulic pump.
Finally, the actual-pressure value may also be determined from the pressure in the servo-cylinder for adjusting the hydraulic pump and from the number of revolutions of the internal combustion engine. The way of determination may also be chosen different depending on the operating state which exists, e.g. depending on the number of revolutions.
As was already explained previously the inventive apparatus permits to limit the pressure in the hydrostatic transmission, thus avoiding an overload of the internal combustion engine. For instance, this is necessary if the internal combustion engine runs up and a relatively high motive force has been required already. In this respect, an aspect of the invention provides that a controllable limitation stage is interposed between the desired-value transducer and the desired-value and actual-value comparator of the pressure control circuit, a second characteristics stage in which an rpm-torque characteristic of the internal combustion engine is stored and provides an output signal to the pressure relief stage, and the inlet is connected to an rpm transducer such that the pressure relief stage presets a pressure signal corresponding to the admissible torque according to a momental value of the characteristics stage and the level of the varying signal for the pump. The characteristic in the second characteristics stage may be obtained from a conversion of the characteristic(s) in the first characteristics stage and the corresponding hydraulic pressure and the motive force may be calculated via the actual delivery volume of the hydraulic pump in order to avoid stalling the internal combustion engine. The desired-value and actual-value transducer, for instance, determines the desired-pressure value for the pressure control circuit from the value desired for the motive force, the radius of the vehicle wheel, and the volume displaced by the hydraulic engine.
It also was set forth previously that the internal combustion engine may drive a second pump having a constant or variable delivery volume for a hydraulic circuit, this having no adverse effect on the efficiency of the inventive apparatus.